The present invention generally relates to a driving circuit for a simple matrix type liquid crystal display apparatus of multiduty, which sequentially outputs, respectively, the picture element data of one line portion through the segment driver into one electrode contacting the liquid crystal, scanning pulses through the common driver into the other electrode contacting the liquid crystal, reversing the polarities of the outputs at a constant period with an alternating signal so as to drive the respective liquid crystal.
Generally, in the simple matrix type liquid crystal display apparatus of multiduty, electrolysis is caused in the liquid crystal. This occurs when the direct current voltage is continuously applied upon the liquid crystal which becomes each picture element, to shorten the service life. The polarities of the voltage signals corresponding to the picture element data and the scanning pulses are reversed and applied upon one or the other electrodes contacting liquid crystal for each of given periods (for example, one frame scanning time) by the alternating signals, so as to turn the average voltage to be applied upon the liquid crystal to zero.
The above described alternating signal is conventionally generated by an alternating signal generating circuit composed of a NAND gate 1 and a D-flip-flop 2 shown in FIG. 4. Namely, the NAND gate 1 receives a frame scanning start signal YD (see FIG. 5(a)) to be inputted at each scanning start of one frame, and the latch pulse LP (see FIG. 5 (b)) to be inputted at each latching of the picture element data of one line portion, to reverse the logical product of both the signals for outputting the set signal D (see FIG. 5(c)) into the D-flip-flop 2. The D-flip-flop 2 outputs into an output terminal Q through a reset terminal D the signal of a reversion output terminal Q at each rising of the set signal D, to get such an alternating signal FR as shown in FIG. 5(d).
FIG. 6 shows a driving circuit for a simple matrix type liquid crystal display apparatus of the conventional multiduty type, adopting a system of reversing the polarities of the picture element data, scanning pulses by the above described alternating signal FR. The driving circuit is composed of a segment driver 12 for sequentially outputting, one line portion, by one line portion the picture element data into the one longitudinal direction of a seven line shaped electrode contacting the liquid crystal matrix 11 which composes the thirty five picture elements of 5.times.7, and a common driver 13 for sequentially outputting in a row direction the scanning pulses into the other lateral direction five line shaped electrode contacting above described the liquid crystal. The binary ("0", "1") picture element data D1, . . ., D7 of seven per line are synchronized with a clock XCK, is accommodated into the shift register 14 of the segment driver 12, and is retained for one horizontal scanning period into a data latch 15 receiving the latch pulse LP. A level converter 16 and an analog switch 17 composed of many transistors adjusted in level, convert seven input signals of "0" or "1" from the data latch 15 into four operation voltage values V5, V0, V3, V2 to be selected by the voltage value selection switches 18, 19 in accordance with the "H", "L" of the alternating signal FR by on signal and off signal so as to effect parallel output into the one seven line shaped electrodes.
The shift register 20 of the common driver 13 synchronizes, to the latch pulse LP to be inputted for each one horizontal scanning operation, the frame scanning start signal YD to be inputted at the scanning start time of one frame, so as to shift it. The data latch 21 retains the shifted pulse signal for one horizontal scanning period. The level converter 22 and the analog switch 23 composed of many transistors adjusted in level, convert the five input signals of "0" or "1" from the data latch 21 into four operation voltage values V0, V5, V4, V1 to be selected, respectively, by the voltage value selection switches 24, 25 in accordance with the "H", "L" of the alternating signal FR from the on signal and off signal so as to effect parallel output into the other five line shaped electrodes. Namely, the scanning pulses V0, V5 are sequentially scanned from top to bottom like Ca, Cb, . . . into the five lateral direction line shaped electrodes by the common driver 13. Among the line liquid crystal with the scanning pulses being fed into it, only the liquid crystal with the white level signals V5, V0 being fed into it by the segment driver 12 is displayed.
FIG. 7 is a view showing across a period of two frames, the electrode wave forms to be outputted respectively from the above-described analog switches 17, 23 into both electrodes contacting the electrode by way of the liquid crystal of Cb line S2 row on the picture face. As shown in FIG. 7 (a), (b), (c), the frame scanning start signal YD is outputted as a pulse signal into each frame scanning start time. The latch pulse LP is outputted five times, setting each horizontal scanning in one frame. The D-flip-flop 2 outputs the alternating signal FR which is varied into "H", "L" for each one frame as described hereinabove (see FIGS. 4, 5) in accordance with both the pulses. The analog switch 17 on the side of the segment driver 12 outputs a black level voltage signal of V3 when the FR is "H" as shown in FIG. 7(d); a black level voltage signal of V2 when the FR is "L"; outputs a white level voltage signal of V5 when the FR is "H"; a white level voltage signal of V0 when the FR is "L". In the shown example, the respective No. 2 line, No. 4 line of the first and second frames become white levels. As shown in FIG. 7(e), the analog switch 23 on the side of the common driver 13 outputs a non-selection level voltage signal of V4 when the FR is the "H", a non-selection level voltage signal of V1 when the FR is the "L", outputs a selection level voltage signal of V0 when the FR is the "H", and a selection level voltage signal of V5 when the FR is "L". In the shown example, the Cb line becomes the selection level, being set to the second latch pulse LP for a period of scanning the Cb line which is the No. 2 line in the first and second frames. Since the voltage signals like FIG. 7 (d), (e) are applied respectively upon both the electrodes contacting the liquid crystal of the Cb line, the S2 row, the potential difference to be added to the liquid crystal is provided as in FIG. 7(f). The above described liquid crystal is too light (display) with voltage .vertline.V0-V5.vertline. only at the No. 2 line scanning time of the first frame, and too light with the negative voltage--.vertline.V0-V5.vertline. only at the second line scanning time of the second frame.
In the above described conventional driving circuit, the picture face of the liquid crystal display apparatus becomes larger, and the wave forms of the output voltage signal from both the drivers 12, 13 shown in FIG. 7 (f) become duller when the liquid crystal picture elements to be driven are increased to increase the load. The duller wave forms appear considerably (see FIG. 8(c)) when the alternating signal FR changes from the "L" to the "H" or from the "H" to the "L" as shown in FIG. 8(d). The output voltage signal from the common driver 13 is much duller. Namely, when the alternating signal FR changes from the "L" to the "H", the non-selection voltage level to be outputted from the common driver 13 changes from V1 to V4. At the same time, after the voltage Ca of the No. 1 line has become V0 in the selection level for only the horizontal scanning period, it returns to the V4 in the non-selection level. Since the voltage variation from the V1 of the voltage Ca to the V0 is small, the circular arc shaped dullness of the wave form corner portion is small, and the voltage variation from the V0 to the V4 is larger, so that the circular arc dullness of the wave form corner portion is larger. Similarly, the voltage Cb of the No. 2 line and the voltage Ce of the No. 5 line are also varied as shown, with the dullness of the wave form corner portion becoming larger as compared with the voltage variation. The black level voltage Lb to be outputted from the segment driver 12 is changed from the V2 to the V3 in accordance with the variation of the FR so as to cause the slight dullness in the wave form corner portion. Therefore, the liquid crystal driving voltage which is the output potential difference of the common driver 13 and the segment driver 12, at the variation time of the alternating signal FR, works in a direction of lowering the potential difference with the dullness of the Ca and the Lb being mutually opposite in direction on the No. 1 line. It is not lowered so much in the potential difference with the dullness of the Cb and the Lb being mutually the same in direction on the No. 2 line. Further, it works in a direction of increasing the potential difference with the dullness of the Ce and the Lb being opposite in direction on the No. 5 line. Thus, the brightness of the line accompanied by the variation in the FR is the darkest in the No. 1 line and is the brightest in the No. 5 line. Thus, the horizontal string shows a constant value of the turbulent picture face of the effective value of the driving signal affected by the variation in the alternating signal FR, with a defect that the display quality is degraded.